A rapid, selective, sensitive, and specific method is presented to simultaneously quantify morphine, 6-monoacetylmorphine (6-MAM), codeine, heroin, fentanyl, and methadone in human whole blood and plasma. The drugs were extracted with phosphate buffer at pH 6, followed by solid-phase extraction (SPE) and quantification by GC/MS with electron impact ionization using helium as carrier gas. Quantification was performed based on nalorphine as internal standard (IS). The specificity, linearity, intra- and inter-assay precision and accuracy, and extraction recovery were fully evaluated. The limits of detection (LODs) were 0.40 - 7.63 ng/mL for whole blood and 0.80 - 32.00 ng/mL for plasma. The method is fast, simple, and accurate, with the sensitivity and specificity required in forensic and clinical toxicology.